1. Field of Invention
This invention relates to automotive power transmissions, and particularly to infinitely variable hydrostatic transmissions, integrated with mechanical power transfer in such a manner as to achieve lower levels of power loss than can be achieved with current hydrodynamically-based automatic transmissions.
2. Description of the Related Art
In the transmission of power from the engine to the drive wheels in typical automotive applications, some sort of transmission device is required to transmit the rotative motion from a primary power source to a power output at selective transmission ratios. The major difficulty in transmitting power from the primary power source, or engine, to the vehicle drive wheels is that the engine only has a capability of transmitting power, or torque, within a defined operating range above idle speed, whereas the vehicle drive wheel speed must go from zero to maximum. In other words, some mechanism is required such that torque from a rapidly rotating power source can initially be transmitted to a stationary load and thereafter be matched in ratios such that, at maximum engine speed, the vehicle will be travelling at the desired maximum speed.
Furthermore, the transmission device must be capable of being selected to a lower relative drive ratio whenever a heavier load is encountered as when accelerating, going up a steep hill or when pulling an attached trailer.
Historically, many different approaches have been taken to resolve the requirement of matching engine output RPM to the desired vehicle operating speed. The first, and most elementary, was the mechanical clutch and manual-shift transmission, wherein the transmission usually incorporated either three or four stepped gear ratios for forward speeds and one for reverse. Further and continued developments in automotive drive systems saw the introduction of a fluid, coupling, or torque converter, between the clutch and manual-shift transmissions to provide a limited degree of continuous variability and downshifting in response to applied load. Further evolution's of the automotive transmission saw the elimination of the clutch and replacement of the manual-shift transmission by means of automatically engaged clutch packs for each ratio. These clutch packs usually provided essentially the same number of distinct ratios as did the earlier manually operated clutch and transmission but were engaged automatically in response to engine speed and applied load.
The current automatic transmission, however, is, at best, a compromise and has never been able to match the manual clutch and transmission in the level of fuel economy achieved. Inventors, though, have long realized that the key to achieving a greater efficiency in power transmission is by means of a transmission having the characteristic of continuously variable drive ratios at output speeds ranging from zero to maximum in both forward and reverse. Most inventors have further realized that some sort of hydrostatic, versus hydrodynamic, means would likely be the key. To date, however, no method of hydrostatic drive has been able to achieve both the functional characteristics required in an automotive transmission as well as the desired levels of mechanical efficiency.
In Europe, some success has been achieved in deploying a continuously variable belt-drive transmission to achieve the desired functional characteristic of continuous variability as well as achieving levels of fuel efficiency matching that of the manual transmission. However, the variable belt-drive transmissions have not been capable of achieving sufficiently high levels of durability and have, thus, not been used in any except the very smallest of cars. While the continuously variable belt drive might appear to have some application in smaller-type vehicles, some method employing a hydrostatic, or combined hydrostatic and mechanical method, would appear to offer the most promise for general automotive use.